In recent years, with the progress of electronic instruments such as portable telephones and personal computers, secondary batteries used in these instruments have been required to have small size and light weight. High-energy-density secondary batteries that can satisfy the requirements are lithium ion secondary batteries. In the meantime, a secondary battery, such as a lead storage battery or a nickel hydrogen battery, has been used as a large-size and large-capacity power source. A typical example of the large-size and large-capacity power source is a power source for an electric vehicle, a hybrid car, an electric motorbike or a forklift. Recently, developments have been actively made to adopt a lithium ion secondary battery, which is high in energy density. In the developments, increases in the size and the capacity thereof have been advanced while an improvement in the cycle life, the safety and others has been taken into consideration.
As a power source for these vehicles or articles, use is made of a battery pack in which plural batteries connected in series or in parallel are held since the driving power thereof is large (see, for example, JP-A 2009-87542 (KOKAI), and JP-A 2009-87720 (KOKAI)).
The shape of sealed secondary batteries is generally a cylindrical shape or a rectangular shape. Attention has been paid, in particular, to rectangular sealed secondary batteries since the batteries are excellent in space efficiency when the batteries are each set into an instrument.
In a sealed secondary battery, use is made of, for example, a flat-shaped electrode group in which band-form positive and negative electrodes, in each of which an electrode active material layer is formed on each surface of a metal foil, are wound into a flat form with a band-form separator interposed therebetween. In order to take out electric energy generated by the flat-shaped electrode group, it is known that a metal foil area on which no electrode active material layer is formed is formed onto each of the positive and negative electrodes, and a lead or the like is joined to each of the areas to take out electric energy. Furthermore, for example, Japanese Patent No. 4134521, and JP-A 2003-197174 (KOKAI) disclose that individual metal foils are laminated/bundled on/with each other, and then leads or the like are welded to the foils in this state in order to enhance the current collecting efficiency.
In order to enhance the energy density of a battery, it is necessary that a larger volume of a flat-shaped electrode group be stored in a metal case. Moreover, it is feared that a large current flows into leads for collecting electric energy or sections for joining the leads, whereby heat is easily generated and at worst the battery may be damaged by the heat.
However, the batteries described in Japanese Patent No. 4134521, and JP-A 2003-197174 (KOKAI) do not sufficiently solve these problems.